In order to analyze subpopulations of cells that have been cocultured in vitro using highly sensitive molecular techniques such as RNA arbitrarily primed PCR (polymerase chain reaction), fluorescence-activated cell separation (in combination with magnetic cell separation if necessary) is used to sort tumor and stromal cells with a high purity. The cooperation with the National Flow Cytometry Resource is of particular importance since I can ask for advice if problems with the flow cytometers and lasers at the Institute of Pathology in Regensburg, Germany, occur. Also, a preliminary study to test the feasability of PKH-(Paul K. Horan)-markers for cocultivation followed by fluorescence-activated cell separation has been carried out at the Los Alamos National Laboratory. A number of investigations in progress involve flow cytometry such as tumor cell cycle analyses in cocultures as opposed to monocultures. The suggestions by members of the National Flow Cytometry Resource in Los Alamos greatly support this work of my group in Regensburg.